Switching power converters are increasingly used in place of analog power amplifiers in high efficiency applications, including, for example, DC-to-DC converters (e.g., voltage regulators), class D power amplifiers, etc. These converters produce a Pulse Width Modulated (PWM) output signal, which generally drives an external inductor and capacitor, and require a feedback loop to adjust the duty cycle of the PWM signal and to control the output voltage or current. Digital instead of analog feedback loops are increasingly used, but in many applications they create “tones,” or “limit cycle oscillations,” which is undesirable. For example, in an application, the PWM drives a low pass filter, and the feedback loop monitors the output of the low pass filter, and adjusts the duty cycle of the PWM signal in order to obtain the desired output voltage. In response to the error signal (e.g., the difference between the desired voltage and the actual output voltage) the digital filter generates a value that in conjunction with an ADC (analog-to-digital converter) controls the digital PWM DAC (digital-to-analog converter). In systems where the digital PWM generator is clocked at a frequency that is a multiple of the PWM update rate, the accuracy of the PWM DAC is limited to the ratio of the clock rate to the update rate, which is typically 16 to 256 for 4 to 8 bits of resolution. The limited DAC resolution then limits the ability of the digital filter to correct the output voltage. As a result, the control loop toggles the output of the PWM DAC between several values, which shows up as quantization noise, “tones,” or “limit cycle oscillations,” and tends to get concentrated at specific frequencies (e.g., the passband of the frequency response of the control loop), which is undesirable.
Like reference symbols in the various drawings indicate like elements.